Habitable. It’s just a word, a concept simple enough for anyone to grasp. It
means you can step out of an airlock without a helmet and not die. But there’s
habitable and then there’s habitable. Most of what we find are marginally
habitable worlds. Sure, you can step outside without a helmet and you won’t die.
At least not right away. But that little qualifier doesn’t mean that you want to
live there.

I mean our dear old Earth, paradise that it was, probably only qualified as
habitable for the last eighth of its existence, and even then it went through
some rough spots – even before we arrived on the scene. There are exceptions,
like Eden, Paradise and Kalmar – even Delphi if you ignore the predators – but
most of what I’ve seen are marginal worlds, places where the local life might be
well-adapted and flourishing, but where we would huddle under burning suns, foul
air or angry skies.

The gas giant moons are often the most marginal. They generally fall into two
types: little water worlds that might just have an island or two and small dry
worlds with shallow seas and windblown skies.